The present invention relates to an optical module which uses a semiconductor laser as a light source and includes a wavelength locker and a modulator.
Recent progress of technologies is stimulating commercialization of an optical module which integrates a modulator and a wavelength locker together with a semiconductor laser and serves as a light source. Such an optical module is described in Japanese Patent Laid-Open No. 2007-115933 (reference 1), Japanese Patent Laid-Open No. 2005-085904 (reference 2), Daniel Mahgerefteh, et al., “Tunable Chirp Managed Laser”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 2, pp. 108-110, 2008 (reference 3), and Raj Batra, et al., “Integrable Tunable Transmitter Assembly White Paper”, OIF. (reference 4). For example, an optical semiconductor module described in reference 1 incorporates a semiconductor optical modulator and uses three lenses for optical coupling. An optical module described in reference 2 incorporates an etalon filter and controls the output wavelength of an LD element. This optical module uses a lens as an optical system for optical coupling.
In all the optical modules, however, a laser is optically coupled with a semiconductor modulator via a lens, resulting in a bulky optical system and a large coupling loss. To the contrary, International Publication No. WO 2005/106546 (reference 5) discloses a technique of optically coupling a laser source with an internal optical filter or optical modulator using a waveguide.
In the technique disclosed in reference 5, a ring oscillator (resonator) extracts light with a predetermined wavelength from light emitted by the laser source, and a photodetector detects the intensity of the extracted light. The oscillation wavelength of the laser source is controlled such that the signal strength to be detected by the photodetector is maximized. This makes it difficult to increase the strength of the optical signal to be output to the modulator. In addition, according to the technique of reference 5, the output of light emitted by the laser source changes upon temperature control to control the oscillation wavelength.